The present invention relates to technology for measuring a ground leakage current in an ungrounded DC (Direct Current) power system, and more particularly, to a ground leakage current measurement apparatus and method, which is capable of smoothly measuring ground leakage currents at both sides although a ground fault simultaneously occurs in positive and negative electric lines in a live wire state or ground faults having the same ground resistance simultaneously occur.
In an ungrounded DC power system, an apparatus for measuring a ground leakage current of an electric line in a live wire state is mainly used for alarming a ground fault and installed in a main power booth. Furthermore, a ground resistance measuring instrument may be installed in the main power booth.
In general, a ground fault alarm circuit employs a voltage divider circuit system using two resistors R1 and R2 as shown in FIG. 1 (hereinafter, referred to as ‘2R system’), and a ground resistance measuring circuit employs a system for generating a measurement voltage by using two Zener diodes ZD1 and ZD2 as shown in FIG. 2 (hereinafter, referred to as ‘2ZD system’).
Both systems will be described more specifically as follows. First, a case in which ground faults having the same ground resistance value occur in both electric lines may be considered in the 2R system. In this case, a divided voltage across both ground fault alarms V1 and V2 for detecting a displacement voltage is not changed in the 2R system. Therefore, the ground fault alarm circuit cannot be operated.
Furthermore, a case in which ground faults having different ground resistance values occur in both electric lines may be considered in the 2R system. In this case, since a ground potential is decided by parallel combined resistance values R1//RG1 and R2//RG2 in the positive and negative sides, alarm operating points of the ground fault alarms V1 and V2 may differ from initial set values. Here, the alarm operating point indicates a critical value which is preset on the assumption that a ground fault occurs only in one electric line.
Furthermore, even when a ground fault occurs only in one electric line in the 2R system, it is impossible to measure a linear displacement voltage depending on variations in ground resistance value. For example, when a positive side ground resistor RG1 varies from 10 kΩ to 20 kΩ in a state in which the resistors R1 and R2 are set to 1 kΩ in FIG. 1, a parallel combined resistance value R1//RG2 in the case of 10 kΩ is about 0.91 kΩ, and a parallel combined resistance value R1//RG2 in the case of 20 kΩ is about 0.95 kΩ. That is, while the value of the ground resistor RG1 doubles, the parallel combined resistance value varies about 1.04 times (0.95/0.91), which means that the variation is non-linear.
Meanwhile, a case in which ground faults having the same ground resistance value occur in both electric lines may be considered in the 2ZD system. When it is assumed that a voltage between the electric lines is 48V, a reference voltage of the Zener diodes ZD1 and ZD2 is 20V, and the ground resistors RG1 and RG2 have the same value, a voltage of 24V is applied to each ground resistor. In this case, a voltage measured by ground leakage current testers A1 and A2 is 4V. Therefore, the ground leakage current testers A1 and A2 measure a different value from an actual ground resistance value.
Furthermore, a case in which ground faults having different ground resistance values occur in both electric lines may be considered in the 2ZD system. At this time, when it is assumed that a voltage between the electric lines is 48V, a reference voltage of the Zener diodes ZD1 and ZD2 is 20V, the ground resistor RG1 has a resistance value of 10 kΩ, and the ground resistor RG2 has a resistance value of 20 kΩ, a voltage across both ends of the ground resistor RG1 is 16V. Therefore, an actually measured voltage is reduced to 4V. Furthermore, since a voltage across both ends of the ground resistor RG2 is 32V, an actually measured voltage becomes a value smaller than zero. Therefore, it is impossible to measure a ground fault in the negative side.
In the 2ZD system, even when a ground fault occurs only in one electric line, for example, in the positive side (RG1=10 kΩ), a current flowing in the ground leakage current tester A1 is 0.4 mA (=4V/10 kΩ). Here, since the reference voltage is 20V, the ground resistance value is measured at 500 kΩ, not 10 kΩ.
Meanwhile, a distributed capacitive component exists between an electric line and the ground, and a capacitive component such as a noise removal capacitor installed in a load exists. As well known, this capacitive component acts as noise during the ground leakage current measurement, and may serve as a cause which makes it difficult to measure an accurate ground resistance value. Furthermore, in order to detect a ground fault position, a method in which a supervisory signal of a signal generator is periodically transmitted to an electric line and a zero phase sequence current transformer (ZCT) installed in a diverging point of the electric line detects a ground leakage current of the diverging point based on the supervisory signal is widely used. When the supervisory signal transmitted to the electric line is a sine wave signal, a capacitive ground leakage current has a phase which leads the phase of a resistive ground leakage current by 90 degrees. When the supervisory signal is a square wave signal, the capacitive ground leakage current exhibits a spike peak. That is, in order to more accurately measure a ground leakage current, the influence of the signal generator as well as the influence of the above-described capacitive components should be removed.